Inkjet Thermal Actuator With Parallel Current Paths

ABSTRACT

An inkjet printhead comprising: an array of ink chambers, each having a nozzle and a thermal actuator for generating vapour bubbles to eject ink through the nozzle; wherein, the thermal actuator has a pair of contacts and at least two parallel current paths between the contacts, each of the current paths having a plurality of heater elements for nucleating a vapour bubble.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation Application of U.S.application Ser. No. 11/246,675 filed on Oct. 11, 2005, the content ofwhich is incorporated herein by cross-reference.

FIELD OF THE INVENTION

The present invention relates to the field of micro-electromechanicalsystems (MEMS) devices and discloses an inkjet printing system usingMEMS techniques.

CROSS REFERENCES TO RELATED APPLICATIONS

Various methods, systems and apparatus relating to the present inventionare disclosed in the following US patents/patent applications filed bythe applicant or assignee of the present invention:

6,750,901 6,476,863 6,788,336 7,249,108 6,566,858 6,331,946 6,246,9706,442,525 7,346,586 09/505,951 6,374,354 7,246,098 6,816,968 6,757,8326,334,190 6,745,331 7,249,109 7,197,642 7,093,139 7,509,292 10/636,28310/866,608 7,210,038 7,401,223 10/940,653 10/942,858 7,364,256 7,258,4177,293,853 7,328,968 7,270,395 7,461,916 7,510,264 7,334,864 7,255,4197,284,819 7,229,148 7,258,416 7,273,263 7,270,393 6,984,017 7,347,5267,357,477 7,465,015 7,364,255 7,357,476 11/003,614 7,284,820 7,341,3287,246,875 7,322,669 7,506,958 7,472,981 7,448,722 7,575,297 7,438,3817,441,863 7,438,382 7,425,051 7,399,057 11/246,671 11/246,670 11/246,6697,448,720 7,448,723 7,445,310 7,399,054 7,425,049 7,367,648 7,370,9367,401,886 7,506,952 7,401,887 7,384,119 7,401,888 7,387,358 7,413,2816,623,101 6,406,129 6,505,916 6,457,809 6,550,895 6,457,812 7,152,9626,428,133 7,204,941 7,282,164 7,465,342 7,278,727 7,417,141 7,452,9897,367,665 7,138,391 7,153,956 7,423,145 7,456,277 7,550,585 7,122,0767,148,345 11/172,816 7,470,315 7,572,327 7,416,280 7,252,366 7,488,0517,360,865 6,746,105 11/246,687 11/246,718 7,322,681 11/246,68611/246,703 11/246,691 7,510,267 7,465,041 11/246,712 7,465,032 7,401,8907,401,910 7,470,010 11/246,702 7,431,432 7,465,037 7,445,317 7,549,73511/246,674 11/246,667 7,156,508 7,159,972 7,083,271 7,165,834 7,080,8947,201,469 7,090,336 7,156,489 7,413,283 7,438,385 7,083,257 7,258,4227,255,423 7,219,980 10/760,253 7,416,274 7,367,649 7,118,192 10/760,1947,322,672 7,077,505 7,198,354 7,077,504 10/760,189 7,198,355 7,401,8947,322,676 7,152,959 7,213,906 7,178,901 7,222,938 7,108,353 7,104,6297,303,930 7,401,405 7,464,466 7,464,465 7,246,886 7,128,400 7,108,3556,991,322 7,287,836 7,118,197 7,575,298 7,364,269 7,077,493 6,962,40210/728,803 7,147,308 7,524,034 7,118,198 7,168,790 7,172,270 7,229,1556,830,318 7,195,342 7,175,261 7,465,035 7,108,356 7,118,202 7,510,2697,134,744 7,510,270 7,134,743 7,182,439 7,210,768 7,465,036 7,134,7457,156,484 7,118,201 7,111,926 7,431,433 7,018,021 7,401,901 7,468,13911/188,017 11/097,308 7,448,729 7,246,876 7,431,431 7,419,249 7,377,6237,328,978 7,334,876 7,147,306 09/575,197 7,079,712 6,825,945 7,330,9746,813,039 6,987,506 7,038,797 6,980,318 6,816,274 7,102,772 7,350,2366,681,045 6,728,000 7,173,722 7,088,459 09/575,181 7,068,382 7,062,6516,789,194 6,789,191 6,644,642 6,502,614 6,622,999 6,669,385 6,549,9356,987,573 6,727,996 6,591,884 6,439,706 6,760,119 7,295,332 6,290,3496,428,155 6,785,016 6,870,966 6,822,639 6,737,591 7,055,739 7,233,3206,830,196 6,832,717 6,957,768 7,456,820 7,170,499 7,106,888 7,123,23910/727,181 10/727,162 7,377,608 7,399,043 7,121,639 7,165,824 7,152,94210/727,157 7,181,572 7,096,137 7,302,592 7,278,034 7,188,282 10/727,15910/727,180 10/727,179 10/727,192 10/727,274 10/727,164 7,523,1117,573,301 10/727,158 10/754,536 10/754,938 10/727,160 10/934,7207,171,323 7,369,270 6,795,215 7,070,098 7,154,638 6,805,419 6,859,2896,977,751 6,398,332 6,394,573 6,622,923 6,747,760 6,921,144 10/884,8817,092,112 7,192,106 7,457,001 7,173,739 6,986,560 7,008,033 7,551,3247,195,328 7,182,422 7,374,266 7,427,117 7,448,707 7,281,330 10/854,5037,328,956 10/854,509 7,188,928 7,093,989 7,377,609 10/854,495 10/854,49810/854,511 7,390,071 10/854,525 10/854,526 7,549,715 7,252,35310/854,515 7,267,417 10/854,505 7,517,036 7,275,805 7,314,261 7,281,7777,290,852 7,484,831 10/854,523 10/854,527 7,549,718 10/854,52010/854,514 7,557,941 10/854,499 10/854,501 7,266,661 7,243,19310/854,518 10/934,628 7,163,345 7,448,734 7,425,050 7,364,263 7,201,4687,360,868 7,234,802 7,303,255 7,287,846 7,156,511 10/760,264 7,258,4327,097,291 10/760,222 10/760,248 7,083,273 7,367,647 7,374,355 7,441,8807,547,092 10/760,206 7,513,598 10/760,270 7,198,352 7,364,264 7,303,2517,201,470 7,121,655 7,293,861 7,232,208 7,328,985 7,344,232 7,083,27211/014,764 11/014,763 7,331,663 7,360,861 7,328,973 7,427,121 7,407,2627,303,252 7,249,822 7,537,309 7,311,382 7,360,860 7,364,257 7,390,0757,350,896 7,429,096 7,384,135 7,331,660 7,416,287 7,488,052 7,322,6847,322,685 7,311,381 7,270,405 7,303,268 7,470,007 7,399,072 7,393,07611/014,750 7,588,301 7,249,833 7,524,016 7,490,927 7,331,661 7,524,0437,300,140 7,357,492 7,357,493 7,566,106 7,380,902 7,284,816 7,284,8457,255,430 7,390,080 7,328,984 7,350,913 7,322,671 7,380,910 7,431,4247,470,006 7,585,054 7,347,534 7,441,865 7,469,989 7,367,650

The disclosures of these applications and patents are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The present invention involves the ejection of ink drops by way offorming gas or vapor bubbles in a bubble forming liquid. This principleis generally described in U.S. Pat. No. 3,747,120 (Stemme). Each pixelin the printed image is derived ink drops ejected from one or more inknozzles. In recent years, inkjet printing has become increasing popularprimarily due to its inexpensive and versatile nature. Many differentaspects and techniques for inkjet printing are described in detail inthe above cross referenced documents.

One of the perennial problems with inkjet printing is the control ofdrop trajectory as it is ejected from the nozzle. With every nozzle,there is a degree of misdirection in the ejected drop. Depending on thedegree of misdirection, this can be detrimental to print quality.

SUMMARY OF THE INVENTION

According to an aspect of the present invention there is provided aninkjet printhead comprising an array of ink chambers, each ink chambercomprising:

a plurality of nozzles; and

a thermal actuator for generating vapour bubbles to eject ink throughthe nozzles, the thermal actuator comprising a pair of contacts and atleast two parallel current paths between the contacts, each of thecurrent paths having a plurality of heater elements connected in series.

Other aspects are also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described byway of example only with reference to the accompanying drawings, inwhich:

FIG. 1 shows a partially fabricated unit cell of the MEMS nozzle arrayon a printhead according to the present invention, the unit cell beingsection along A-A of FIG. 3;

FIG. 2 shows a perspective of the partially fabricated unit cell of FIG.1;

FIG. 3 shows the mark associated with the etch of the heater elementtrench;

FIG. 4 is a sectioned view of the unit cell after the etch of thetrench;

FIG. 5 is a perspective view of the unit cell shown in FIG. 4;

FIG. 6 is the mask associated with the deposition of sacrificialphotoresist shown in FIG. 7;

FIG. 7 shows the unit cell after the deposition of sacrificialphotoresist trench, with partial enlargements of the gaps between theedges of the sacrificial material and the side walls of the trench;

FIG. 8 is a perspective of the unit cell shown in FIG. 7;

FIG. 9 shows the unit cell following the reflow of the sacrificialphotoresist to close the gaps along the side walls of the trench;

FIG. 10 is a perspective of the unit cell shown in FIG. 9;

FIG. 11 is a section view showing the deposition of the heater materiallayer;

FIG. 12 is a perspective of the unit cell shown in FIG. 11;

FIG. 13 is the mask associated with the metal etch of the heatermaterial shown in FIG. 14;

FIG. 14 is a section view showing the metal etch to shape the heateractuators;

FIG. 15 is a perspective of the unit cell shown in FIG. 14;

FIG. 16 is the mask associated with the etch shown in FIG. 17;

FIG. 17 shows the deposition of the photoresist layer and subsequentetch of the ink inlet to the passivation layer on top of the CMOS drivelayers;

FIG. 18 is a perspective of the unit cell shown in FIG. 17;

FIG. 19 shows the oxide etch through the passivation and CMOS layers tothe underlying silicon wafer;

FIG. 20 is a perspective of the unit cell shown in FIG. 19;

FIG. 21 is the deep anisotropic etch of the ink inlet into the siliconwafer;

FIG. 22 is a perspective of the unit cell shown in FIG. 21;

FIG. 23 is the mask associated with the photoresist etch shown in FIG.24;

FIG. 24 shows the photoresist etch to form openings for the chamber roofand side walls;

FIG. 25 is a perspective of the unit cell shown in FIG. 24;

FIG. 26 shows the deposition of the side wall and risk material;

FIG. 27 is a perspective of the unit cell shown in FIG. 26;

FIG. 28 is the mask associated with the nozzle rim etch shown in FIG.29;

FIG. 29 shows the etch of the roof layer to form the nozzle aperturerim;

FIG. 30 is a perspective of the unit cell shown in FIG. 29;

FIG. 31 is the mask associated with the nozzle aperture etch shown inFIG. 32;

FIG. 32 shows the etch of the roof material to form the ellipticalnozzle apertures;

FIG. 33 is a perspective of the unit cell shown in FIG. 32;

FIG. 34 shows the oxygen plasma release etch of the first and secondsacrificial layers;

FIG. 35 is a perspective of the unit cell shown in FIG. 34;

FIG. 36 shows the unit cell after the release etch, as well as theopposing side of the wafer;

FIG. 37 is a perspective of the unit cell shown in FIG. 36;

FIG. 38 is the mask associated with the reverse etch shown in FIG. 39;

FIG. 39 shows the reverse etch of the ink supply channel into the wafer;

FIG. 40 is a perspective of unit cell shown in FIG. 39;

FIG. 41 shows the thinning of the wafer by backside etching;

FIG. 42 is a perspective of the unit cell shown in FIG. 41;

FIG. 43 is a partial perspective of the array of nozzles on theprinthead according to the present invention;

FIG. 44 shows the plan view of a unit cell;

FIG. 45 shows a perspective of the unit cell shown in FIG. 44;

FIG. 46 is schematic plan view of two unit cells with the roof layerremoved but certain roof layer features shown in outline only;

FIG. 47 is schematic plan view of two unit cells with the roof layerremoved but the nozzle openings shown in outline only;

FIG. 48 is a partial schematic plan view of unit cells with ink inletapertures in the sidewall of the chambers;

FIG. 49 is schematic plan view of a unit cells with the roof layerremoved but the nozzle openings shown in outline only;

FIG. 50 is a partial plan view of the nozzle plate with stictionreducing formations and a particle of paper dust;

FIG. 51 is a partial plan view of the nozzle plate with residual inkgutters;

FIG. 52 is a partial section view showing the deposition of SAC1photoresist in accordance with prior art techniques used to avoidstringers;

FIG. 53 is a partial section view showing the deposition of a layer ofheater material onto the SAC1 photoresist scaffold deposited in FIG. 52;

FIG. 54 is a partial schematic plan view of a unit cell with multiplenozzles and actuators in each of the chambers;

FIGS. 55 to 59 are schematic cross sections of the ink chamber shown inFIG. 44 at sequential stages of drop ejection;

FIG. 60 is a schematic perspective of a nozzle with droplet stem anchoras shown in FIG. 61;

FIG. 61 is a plan view of nozzle apertures with centrally disposeddroplet stem anchors;

FIG. 62 is schematic plan view of a unit cell with the roof layerremoved showing a simple ‘theta’ heater element;

FIG. 63 shows a theta heater element with a sudden reduction in crosssection on the cross bar to locate the droplet stem;

FIG. 64 shows a theta heater element with a formation in cross sectionon the cross bar to locate the droplet stem;

FIG. 65 shows a dual bar, four kink heater element;

FIG. 66 is schematic plan view of a unit cell with a Tesla valve torectify the ink flow through the chamber inlets; and,

FIG. 67 is a schematic perspective of a nozzle with a spur extendinginto the nozzle aperture for controlled drop misdirection.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the description than follows, corresponding reference numerals relateto corresponding parts. For convenience, the features indicated by eachreference numeral are listed below.

-   1. Nozzle Unit Cell-   2. Silicon Wafer-   3. Topmost Aluminium Metal Layer in the CMOS metal layers-   4. Passivation Layer-   5. CVD Oxide Layer-   6. Ink Inlet Opening in Topmost Aluminium Metal Layer 3.-   7. Pit Opening in Topmost Aluminium Metal Layer 3.-   8. Pit-   9. Electrodes-   10. SAC1 Photoresist Layer-   11. Heater Material (TiAlN)-   12. Thermal Actuator-   13. Photoresist Layer-   14. Ink Inlet Opening Etched Through Photo Resist Layer-   15. Ink Inlet Passage-   16. SAC2 Photoresist Layer-   17. Chamber Side Wall Openings-   18. Front Channel Priming Feature-   19. Barrier Formation at Ink Inlet-   20. Chamber Roof Layer-   21. Roof-   22. Sidewalls-   23. Ink Conduit-   24. Nozzle Chambers-   25. Elliptical Nozzle Rim    -   25(a) Inner Lip    -   25(b) Outer Lip-   26. Nozzle Aperture-   27. Ink Supply Channel-   28. Contacts-   29. Heater Element.-   30. Bubble cage-   32. bubble retention structure-   34. ink permeable structure-   36. bleed hole-   38. ink chamber-   40. dual row filter-   42. paper dust-   44. ink gutters-   46. gap between SAC1 and trench sidewall-   48. trench sidewall-   50. raised lip of SAC1 around edge of trench-   52. thinner inclined section of heater material-   54. cold spot between series connected heater elements-   56. nozzle plate-   58. columnar projections-   60. sidewall ink opening-   62. ink refill opening-   64. ink-   66. bubble-   68. bulging ink meniscus-   70. ink bulb-   72. droplet stem-   74. droplet stem attachment point-   76. nozzle centre-line-   78. drop misdirection-   80. drop-   82. satellite drop-   84. droplet stem anchor-   86. maximum resistance section or ‘hotspot’-   88. shots either side of droplet stem anchor-   90. semi-circular current path-   92. ‘cold spot’-   94. central bar-   96. larger radius curve-   98. tight radius curve-   100. outside edge of tight radius curve-   102. inside edge of tight radius curve-   104. ink refill aperture-   106. rectifying valve (Tesla valve)-   108. main conduit-   110. secondary conduit-   112. lateral spur from nozzle rim

MEMS Manufacturing Process

The MEMS manufacturing process builds up nozzle structures on a siliconwafer after the completion of CMOS processing. FIG. 2 is a cutawayperspective view of a nozzle unit cell 1 after the completion of CMOSprocessing and before MEMS processing.

During CMOS processing of the wafer, four metal layers are depositedonto a silicon wafer 2, with the metal layers being interspersed betweeninterlayer dielectric (ILD) layers. The four metal layers are referredto as M1, M2, M3 and M4 layers and are built up sequentially on thewafer during CMOS processing. These CMOS layers provide all the drivecircuitry and logic for operating the printhead.

In the completed printhead, each heater element actuator is connected tothe CMOS via a pair of electrodes defined in the outermost M4 layer.Hence, the M4 CMOS layer is the foundation for subsequent MEMSprocessing of the wafer. The M4 layer also defines bonding pads along alongitudinal edge of each printhead integrated circuit. These bondingpads (not shown) allow the CMOS to be connected to a microprocessor viawire bonds extending from the bonding pads.

FIGS. 1 and 2 show the aluminium M4 layer 3 having a passivation layer 4deposited thereon. (Only MEMS features of the M4 layer are shown inthese Figures; the main CMOS features of the M4 layer are positionedoutside the nozzle unit cell). The M4 layer 3 has a thickness of 1micron and is itself deposited on a 2 micron layer of CVD oxide 5. Asshown in FIGS. 1 and 2, the M4 layer 3 has an ink inlet opening 6 andpit openings 7. These openings define the positions of the ink inlet andpits formed subsequently in the MEMS process.

Before MEMS processing of the unit cell 1 begins, bonding pads along alongitudinal edge of each printhead integrated circuit are defined byetching through the passivation layer 4. This etch reveals the M4 layer3 at the bonding pad positions. The nozzle unit cell 1 is completelymasked with photoresist for this step and, hence, is unaffected by theetch.

Turning to FIGS. 3 to 5, the first stage of MEMS processing etches a pit8 through the passivation layer 4 and the CVD oxide layer 5. This etchis defined using a layer of photoresist (not shown) exposed by the darktone pit mask shown in FIG. 3. The pit 8 has a depth of 2 microns, asmeasured from the top of the M4 layer 3. At the same time as etching thepit 8, electrodes 9 are defined on either side of the pit by partiallyrevealing the M4 layer 3 through the passivation layer 4. In thecompleted nozzle, a heater element is suspended across the pit 8 betweenthe electrodes 9.

In the next step (FIGS. 6 to 8), the pit 8 is filled with a firstsacrificial layer (“SAC1”) of photoresist 10. A 2 micron layer of highviscosity photoresist is first spun onto the wafer and then exposedusing the dark tone mask shown in FIG. 6. The SAC1 photoresist 10 formsa scaffold for subsequent deposition of the heater material across theelectrodes 9 on either side of the pit 8. Consequently, it is importantthe SAC1 photoresist 10 has a planar upper surface that is flush withthe upper surface of the electrodes 9. At the same time, the SAC1photoresist must completely fill the pit 8 to avoid ‘stringers’ ofconductive heater material extending across the pit and shorting out theelectrodes 9.

Typically, when filling trenches with photoresist, it is necessary toexpose the photoresist outside the perimeter of the trench in order toensure that photoresist fills against the walls of the trench and,therefore, avoid ‘stringers’ in subsequent deposition steps. However,this technique results in a raised (or spiked) rim of photoresist aroundthe perimeter of the trench. This is undesirable because in a subsequentdeposition step, material is deposited unevenly onto the raisedrim-vertical or angled surfaces on the rim will receive less depositedmaterial than the horizontal planar surface of the photoresist fillingthe trench. The result is ‘resistance hotspots’ in regions wherematerial is thinly deposited.

As shown in FIG. 7, the present process deliberately exposes the SAC1photoresist 10 inside the perimeter walls of the pit 8 (e.g. within 0.5microns) using the mask shown in FIG. 6. This ensures a planar uppersurface of the SAC1 photoresist 10 and avoids any spiked regions ofphotoresist around the perimeter rim of the pit 8.

After exposure of the SAC1 photoresist 10, the photoresist is reflowedby heating. Reflowing the photoresist allows it to flow to the walls ofthe pit 8, filling it exactly. FIGS. 9 and 10 show the SAC1 photoresist10 after reflow. The photoresist has a planar upper surface and meetsflush with the upper surface of the M4 layer 3, which forms theelectrodes 9. Following reflow, the SAC1 photoresist 10 is U.V. curedand/or hardbaked to avoid any reflow during the subsequent depositionstep of heater material.

FIGS. 11 and 12 show the unit cell after deposition of the 0.5 micronsof heater material 11 onto the SAC1 photoresist 10. Due to the reflowprocess described above, the heater material 11 is deposited evenly andin a planar layer over the electrodes 9 and the SAC1 photoresist 10. Theheater material may be comprised of any suitable conductive material,such as TiAl, TiN, TiAlN, TiAlSiN etc. A typical heater materialdeposition process may involve sequential deposition of a 100 Å seedlayer of TiAl, a 2500 Å layer of TiAlN, a further 100 Å seed layer ofTiAl and finally a further 2500 Å layer of TiAlN.

Referring to FIGS. 13 to 15, in the next step, the layer of heatermaterial 11 is etched to define the thermal actuator 12. Each actuator12 has contacts 28 that establish an electrical connection to respectiveelectrodes 9 on either side of the SAC1 photoresist 10. A heater element29 spans between its corresponding contacts 28.

This etch is defined by a layer of photoresist (not shown) exposed usingthe dark tone mask shown in FIG. 13. As shown in FIG. 15, the heaterelement 12 is a linear beam spanning between the pair of electrodes 9.However, the heater element 12 may alternatively adopt otherconfigurations, such as those described in Applicant's U.S. Pat. No.6,755,509, the content of which is herein incorporated by reference. Forexample, heater element 29 configurations having a central void may beadvantageous for minimizing the deleterious effects of cavitation forceson the heater material when a bubble collapses during ink ejection.Other forms of cavitation protection may be adopted such as ‘bubbleventing’ and the use of self passivating materials. These cavitationmanagement techniques are discussed in detail in US patent application(our docket MTC001US).

In the next sequence of steps, an ink inlet for the nozzle is etchedthrough the passivation layer 4, the oxide layer 5 and the silicon wafer2. During CMOS processing, each of the metal layers had an ink inletopening (see, for example, opening 6 in the M4 layer 3 in FIG. 1) etchedtherethrough in preparation for this ink inlet etch. These metal layers,together with the interspersed ILD layers, form a seal ring for the inkinlet, preventing ink from seeping into the CMOS layers.

Referring to FIGS. 16 to 18, a relatively thick layer of photoresist 13is spun onto the wafer and exposed using the dark tone mask shown inFIG. 16. The thickness of photoresist 13 required will depend on theselectivity of the deep reactive ion etch (DRIE) used to etch the inkinlet. With an ink inlet opening 14 defined in the photoresist 13, thewafer is ready for the subsequent etch steps.

In the first etch step (FIGS. 19 and 20), the dielectric layers(passivation layer 4 and oxide layer 5) are etched through to thesilicon wafer below. Any standard oxide etch (e.g. O₂/C₄F₈ plasma) maybe used.

In the second etch step (FIGS. 21 and 22), an ink inlet 15 is etchedthrough the silicon wafer 2 to a depth of 25 microns, using the samephotoresist mask 13. Any standard anisotropic DRIE, such as the Boschetch (see U.S. Pat. Nos. 6,501,893 and 6,284,148) may be used for thisetch. Following etching of the ink inlet 15, the photoresist layer 13 isremoved by plasma ashing.

In the next step, the ink inlet 15 is plugged with photoresist and asecond sacrificial layer (“SAC2”) of photoresist 16 is built up on topof the SAC 1 photoresist 10 and passivation layer 4. The SAC2photoresist 16 will serve as a scaffold for subsequent deposition ofroof material, which forms a roof and sidewalls for each nozzle chamber.Referring to FIGS. 23 to 25, a ˜6 micron layer of high viscosityphotoresist is spun onto the wafer and exposed using the dark tone maskshown in FIG. 23.

As shown in FIGS. 23 and 25, the mask exposes sidewall openings 17 inthe SAC2 photoresist 16 corresponding to the positions of chambersidewalls and sidewalls for an ink conduit. In addition, openings 18 and19 are exposed adjacent the plugged inlet 15 and nozzle chamber entrancerespectively. These openings 18 and 19 will be filled with roof materialin the subsequent roof deposition step and provide unique advantages inthe present nozzle design. Specifically, the openings 18 filled withroof material act as priming features, which assist in drawing ink fromthe inlet 15 into each nozzle chamber. This is described in greaterdetail below. The openings 19 filled with roof material act as filterstructures and fluidic cross talk barriers. These help prevent airbubbles from entering the nozzle chambers and diffuses pressure pulsesgenerated by the thermal actuator 12.

Referring to FIGS. 26 and 27, the next stage deposits 3 microns of roofmaterial 20 onto the SAC2 photoresist 16 by PECVD. The roof material 20fills the openings 17, 18 and 19 in the SAC2 photoresist 16 to formnozzle chambers 24 having a roof 21 and sidewalls 22. An ink conduit 23for supplying ink into each nozzle chamber is also formed duringdeposition of the roof material 20. In addition, any priming featuresand filter structures (not shown in FIGS. 26 and 27) are formed at thesame time. The roofs 21, each corresponding to a respective nozzlechamber 24, span across adjacent nozzle chambers in a row to form acontinuous nozzle plate. The roof material 20 may be comprised of anysuitable material, such as silicon nitride, silicon oxide, siliconoxynitride, aluminium nitride etc.

Referring to FIGS. 28 to 30, the next stage defines an elliptical nozzlerim 25 in the roof 21 by etching away 2 microns of roof material 20.This etch is defined using a layer of photoresist (not shown) exposed bythe dark tone rim mask shown in FIG. 28. The elliptical rim 25 comprisestwo coaxial rim lips 25 a and 25 b, positioned over their respectivethermal actuator 12.

Referring to FIGS. 31 to 33, the next stage defines an elliptical nozzleaperture 26 in the roof 21 by etching all the way through the remainingroof material 20, which is bounded by the rim 25. This etch is definedusing a layer of photoresist (not shown) exposed by the dark tone roofmask shown in FIG. 31. The elliptical nozzle aperture 26 is positionedover the thermal actuator 12, as shown in FIG. 33.

With all the MEMS nozzle features now fully formed, the next stageremoves the SAC1 and SAC2 photoresist layers 10 and 16 by O₂ plasmaashing (FIGS. 34 to 35). After ashing, the thermal actuator 12 issuspended in a single plane over the pit 8. The coplanar deposition ofthe contacts 28 and the heater element 29 provides an efficientelectrical connection with the electrodes 9.

FIGS. 36 and 37 show the entire thickness (150 microns) of the siliconwafer 2 after ashing the SAC1 and SAC2 photoresist layers 10 and 16.

Referring to FIGS. 38 to 40, once frontside MEMS processing of the waferis completed, ink supply channels 27 are etched from the backside of thewafer to meet with the ink inlets 15 using a standard anisotropic DRIE.This backside etch is defined using a layer of photoresist (not shown)exposed by the dark tone mask shown in FIG. 38. The ink supply channel27 makes a fluidic connection between the backside of the wafer and theink inlets 15.

Finally, and referring to FIGS. 41 and 42, the wafer is thinned 135microns by backside etching. FIG. 43 shows three adjacent rows ofnozzles in a cutaway perspective view of a completed printheadintegrated circuit. Each row of nozzles has a respective ink supplychannel 27 extending along its length and supplying ink to a pluralityof ink inlets 15 in each row. The ink inlets, in turn, supply ink to theink conduit 23 for each row, with each nozzle chamber receiving ink froma common ink conduit for that row.

Features and Advantages of Particular Embodiments

Discussed below, under appropriate sub-headings, are certain specificfeatures of embodiments of the invention, and the advantages of thesefeatures. The features are to be considered in relation to all of thedrawings pertaining to the present invention unless the contextspecifically excludes certain drawings, and relates to those drawingsspecifically referred to.

Low Loss Electrodes

As shown in FIGS. 41 and 42, the heater element 29 is suspended withinthe chamber. This ensures that the heater element is immersed in inkwhen the chamber is primed. Completely immersing the heater element inink dramatically improves the printhead efficiency. Much less heatdissipates into the underlying wafer substrate so more of the inputenergy is used to generate the bubble that ejects the ink.

To suspend the heater element, the contacts may be used to support theelement at its raised position. Essentially, the contacts at either endof the heater element can have vertical or inclined sections to connectthe respective electrodes on the CMOS drive to the element at anelevated position. However, heater material deposited on vertical orinclined surfaces is thinner than on horizontal surfaces. To avoidundesirable resistive losses from the thinner sections, the contactportion of the thermal actuator needs to be relatively large. Largercontacts occupy a significant area of the wafer surface and limit thenozzle packing density.

To immerse the heater, the present invention etches a pit or trench 8between the electrodes 9 to drop the level of the chamber floor. Asdiscussed above, a layer of sacrificial photoresist (SAC) 10 (see FIG.9) is deposited in the trench to provide a scaffold for the heaterelement. However, depositing SAC 10 in the trench 8 and simply coveringit with a layer of heater material, can lead to stringers forming in thegaps 46 between the SAC 10 and the sidewalls 48 of the trench 8 (aspreviously described in relation to FIG. 7). The gaps form because it isdifficult to precisely match the mask with the sides of the trench 8.Usually, when the masked photoresist is exposed, the gaps 46 formbetween the sides of the pit and the SAC. When the heater material layeris deposited, it fills these gaps to form ‘stringers’ (as they areknown). The stringers remain in the trench 8 after the metal etch (thatshapes the heater element) and the release etch (to finally remove theSAC). The stringers can short circuit the heater so that it fails togenerate a bubble.

Turning now to FIGS. 52 and 53, the ‘traditional’ technique for avoidingstringers is illustrated. By making the UV mask that exposes the SACslightly bigger than the trench 8, the SAC 10 will be deposited over theside walls 48 so that no gaps form. Unfortunately, this produces araised lip 50 around top of the trench. When the heater material layer11 is deposited (see FIG. 53), it is thinner on the vertical or inclinedsurfaces 52 of the lip 50. After the metal etch and release etch, thesethin lip formations 52 remain and cause ‘hotspots’ because the localizedthinning increases resistance. These hotspots affect the operation ofthe heater and typically reduce heater life.

As discussed above, the Applicant has found that reflowing the SAC 10closes the gaps 46 so that the scaffold between the electrodes 9 iscompletely flat. This allows the entire thermal actuator 12 to beplanar. The planar structure of the thermal actuator, with contactsdirectly deposited onto the CMOS electrodes 9 and suspended heaterelement 29, avoids hotspots caused by vertical or inclined surfaces sothat the contacts can be much smaller structures without acceptableincreases in resistive losses. Low resistive losses preserves theefficient operation of a suspended heater element and the small contactsize is convenient for close nozzle packing on the printhead.

Multiple Nozzles for Each Chamber

Referring to FIG. 49, the unit cell shown has two separate ink chambers38, each chamber having heater element 29 extending between respectivepairs of contacts 28. Ink permeable structures 34 are positioned in theink refill openings so that ink can enter the chambers, but uponactuation, the structures 34 provide enough hydraulic resistance toreduce any reverse flow or fluidic cross talk to an acceptable level.

Ink is fed from the reverse side of the wafer through the ink inlet 15.Priming features 18 extend into the inlet opening so that an inkmeniscus does not pin itself to the peripheral edge of the opening andstop the ink flow. Ink from the inlet 15 fills the lateral ink conduit23 which supplies both chambers 38 of the unit cell.

Instead of a single nozzle per chamber, each chamber 38 has two nozzles25. When the heater element 29 actuates (forms a bubble), two drops ofink are ejected; one from each nozzle 25. Each individual drop of inkhas less volume than the single drop ejected if the chamber had only onenozzle. By ejecting multiple drops from a single chamber simultaneouslyimproves the print quality.

With every nozzle, there is a degree of misdirection in the ejecteddrop. Depending on the degree of misdirection, this can be detrimentalto print quality. By giving the chamber multiple nozzles, each nozzleejects drops of smaller volume, and having different misdirections.Several small drops misdirected in different directions are lessdetrimental to print quality than a single relatively large misdirecteddrop. The Applicant has found that the eye averages the misdirections ofeach small drop and effectively ‘sees’ a dot from a single drop with asignificantly less overall misdirection.

A multi nozzle chamber can also eject drops more efficiently than asingle nozzle chamber. The heater element 29 is an elongate suspendedbeam of TiAlN and the bubble it forms is likewise elongated. Thepressure pulse created by an elongate bubble will cause ink to ejectthrough a centrally disposed nozzle. However, some of the energy fromthe pressure pulse is dissipated in hydraulic losses associated with themismatch between the geometry of the bubble and that of the nozzle.

Spacing several nozzles 25 along the length of the heater element 29reduces the geometric discrepancy between the bubble shape and thenozzle configuration through which the ink ejects. This in turn reduceshydraulic resistance to ink ejection and thereby improves printheadefficiency.

Elliptical Nozzle

Similarly, the hydraulic resistance to droplet ejection can be reducedby using an elliptical nozzle. As shown in FIG. 44, the vapour bubblesgenerated by the heater elements 29 are elongated. The heater elementsare designed to heat uniformly along most of their length so bubblenucleation and growth is likewise substantially uniform along thelength. With an elliptical nozzle 25 centred over the heater element 29such that its major axis is parallel with the centre-line of theelement, the geometry of the bubble roughly corresponds to that of thenozzle. Hence the ink pushed along by the pressure pulse is not changingdirection sharply and generating high fluidic drag before ejectingthrough the nozzle. With less power required for droplet ejection, theprinthead is more efficient.

The elliptical nozzle is also thinner than a circular nozzle ofequivalent aperture area. Hence the spacing between adjacent nozzles isreduced. This helps to increase nozzle pitch and therefore improve printresolution.

Ink Chamber Re-Filled Via Adjacent Ink Chamber

Referring to FIG. 46, two opposing unit cells are shown. In thisembodiment, unit cell has four ink chambers 38. The chambers are definedby the sidewalls 22 and the ink permeable structures 34. Each chamberhas its own heater element 29. The heater elements 29 are arranged inpairs that are connected in series. Between each pair is ‘cold spot’ 54with lower resistance and or greater heat sinking. This ensures thatbubbles do not nucleate at the cold spots 54 and thus the cold spotsbecome the common contact between the outer contacts 28 for each heaterelement pair.

The ink permeable structures 34 allow ink to refill the chambers 38after drop ejection but baffle the pressure pulse from each heaterelement 29 to reduce the fluidic cross talk between adjacent chambers.It will be appreciated that this embodiment has many parallels with thatshown in FIG. 49 discussed above. However, the present embodimenteffectively divides the relatively long chambers of FIG. 49 into twoseparate chambers. This further aligns the geometry of the bubble formedby the heater element 29 with the shape of the nozzle 25 to reducehydraulic losses during drop ejection. This is achieved without reducingthe nozzle density but it does add some complexity to the fabricationprocess.

The conduits (ink inlets 15 and supply conduits 23) for distributing inkto every ink chamber in the array can occupy a significant proportion ofthe wafer area. This can be a limiting factor for nozzle density on theprinthead. By making some ink chambers part of the ink flow path toother ink chambers, while keeping each chamber sufficiently free offluidic cross talk, reduces the amount of wafer area lost to ink supplyconduits.

Ink Chamber with Multiple Actuators and Respective Nozzles

Referring to FIG. 54, the unit cell shown has two chambers 38; eachchamber has two heater elements 29 and two nozzles 25. The effectivereduction in drop misdirection by using multiple nozzles per chamber isdiscussed above in relation to the embodiment shown in FIG. 49. Theadditional benefits of dividing a single elongate chamber into separatechambers, each with their own actuators, is described above withreference to the embodiment shown in FIG. 46. The present embodimentuses multiple nozzles and multiple actuators in each chamber to achievemuch of the advantages of the FIG. 46 embodiment with a markedly lesscomplicated design. With a simplified design, the overall dimensions ofthe unit cell are reduced thereby permitting greater nozzle densities.In the embodiment shown, the footprint of the unit cell is 64 μm long by16 μm wide.

The ink permeable structure 34 is a single column at the ink refillopening to each chamber 38 instead of three spaced columns as with theFIG. 46 embodiment. The single column has a cross section profiled to beless resistive to refill flow, but more resistive to sudden back flowfrom the actuation pressure pulse. Both heater elements in each chambercan be deposited simultaneously, together with the contacts 28 and thecold spot feature 54. Both chambers 38 are supplied with ink from acommon ink inlet 15 and supply conduit 23. These features also allow thefootprint to be reduced and they are discussed in more detail below. Thepriming features 18 have been made integral with one of the chambersidewalls 22 and a wall ink conduit 23. The dual purpose nature of thesefeatures simplifies the fabrication and helps to keep the designcompact.

Multiple Chambers and Multiple Nozzles for Each Drive Circuit

In FIG. 54, the actuators are connected in series and therefore fire inunison from the same drive signal to simplify the CMOS drive circuitry.In the FIG. 46 unit cell, actuators in adjacent nozzles are connected inseries within the same drive circuit. Of course, the actuators inadjacent chambers could also be connected in parallel. In contrast, werethe actuators in each chamber to be in separate circuits, the CMOS drivecircuitry would be more complex and the dimensions of the unit cellfootprint would increase. In printhead designs where the dropmisdirection is addressed by substituting multiple smaller drops,combining several actuators and their respective nozzles into a commondrive circuit is an efficient implementation both in terms of printheadIC fabrication and nozzles density.

High Density Thermal Inkjet Printhead

Reduction in the unit cell width enables the printhead to have nozzlespatterns that previously would have required the nozzle density to bereduced. Of course, a lower nozzle density has a corresponding influenceon printhead size and/or print quality.

Traditionally, the nozzle rows are arranged in pairs with the actuatorsfor each row extending in opposite directions. The rows are staggeredwith respect to each other so that the printing resolution (dots perinch) is twice the nozzle pitch (nozzles per inch) along each row. Byconfiguring the components of the unit cell such that the overall widthof the unit is reduced, the same number of nozzles can be arranged intoa single row instead of two staggered and opposing rows withoutsacrificing any print resolution (d.p.i.). The embodiments shown in theaccompanying figures achieve a nozzle pitch of more than 1000 nozzlesper inch in each linear row. At this nozzle pitch, the print resolutionof the printhead is better than photographic (1600 dpi) when twoopposing staggered rows are considered, and there is sufficient capacityfor nozzle redundancy, dead nozzle compensation and so on which ensuresthe operation life of the printhead remains satisfactory. As discussedabove, the embodiment shown in FIG. 54 has a footprint that is 16 μmwide and therefore the nozzle pitch along one row is about 1600 nozzlesper inch. Accordingly, two offset staggered rows yield a resolution ofabout 3200 d.p.i.

With the realisation of the particular benefits associated with anarrower unit cell, the Applicant has focussed on identifying andcombining a number of features to reduce the relevant dimensions ofstructures in the printhead. For example, elliptical nozzles, shiftingthe ink inlet from the chamber, finer geometry logic and shorter driveFETs (field effect transistors) are features developed by the Applicantto derive some of the embodiments shown. Each contributing featurenecessitated a departure from conventional wisdom in the field, such asreducing the FET drive voltage from the widely used traditional 5V to2.5V in order to decrease transistor length.

Reduced Stiction Printhead Surface

Static friction, or “stiction” as it has become known, allows dustparticles to “stick” to nozzle plates and thereby clog nozzles. FIG. 50shows a portion of the nozzle plate 56. For clarity, the nozzleapertures 26 and the nozzle rims 25 are also shown. The exterior surfaceof the nozzle plate is patterned with columnar projections 58 extendinga short distance from the plate surface. The nozzle plate could also bepatterned with other surface formations such as closely spaced ridges,corrugations or bumps. However, it is easy to create a suitable UV maskfor the pattern columnar projections shown, and it is a simple matter toetch the columns into the exterior surface.

By reducing the co-efficient of static friction, there is lesslikelihood that paper dust or other contaminants will clog the nozzlesin the nozzle plate. Patterning the exterior of the nozzle plate withraised formations limits the surface area that dust particles contact.If the particles can only contact the outer extremities of eachformation, the friction between the particles and the nozzle plate isminimal so attachment is much less likely. If the particles do attach,they are more likely to be removed by printhead maintenance cycles.

Inlet Priming Feature

Referring to FIG. 47, two unit cells are shown extending in oppositedirections to each other. The ink inlet passage 15 supplies ink to thefour chambers 38 via the lateral ink conduit 23. Distributing inkthrough micron-scale conduits, such as the ink inlet 15, to individualMEMS nozzles in an inkjet printhead is complicated by factors that donot arise in macro-scale flow. A meniscus can form and, depending on thegeometry of the aperture, it can ‘pin’ itself to the lip of the aperturequite strongly. This can be useful in printheads, such as bleed holesthat vent trapped air bubbles but retain the ink, but it can also beproblematic if stops ink flow to some chambers. This will most likelyoccur when initially priming the printhead with ink. If the ink meniscuspins at the ink inlet opening, the chambers supplied by that inlet willstay unprimed.

To guard against this, two priming features 18 are formed so that theyextend through the plane of the inlet aperture 15. The priming features18 are columns extending from the interior of the nozzle plate (notshown) to the periphery of the inlet 15. A part of each column 18 iswithin the periphery so that the surface tension of an ink meniscus atthe ink inlet will form at the priming features 18 so as to draw the inkout of the inlet. This ‘unpins’ the meniscus from that section of theperiphery and the flow toward the ink chambers.

The priming features 18 can take many forms, as long as they present asurface that extends transverse to the plane of the aperture.Furthermore, the priming feature can be an integral part of othernozzles features as shown in FIG. 54.

Side Entry Ink Chamber

Referring to FIG. 48, several adjacent unit cells are shown. In thisembodiment, the elongate heater elements 29 extend parallel to the inkdistribution conduit 23. Accordingly, the elongate ink chambers 38 arelikewise aligned with the ink conduit 23. Sidewall openings 60 connectthe chambers 38 to the ink conduit 23. Configuring the ink chambers sothat they have side inlets reduces the ink refill times. The inlets arewider and therefore refill flow rates are higher. The sidewall openings60 have ink permeable structures 34 to keep fluidic cross talk to anacceptable level.

Inlet Filter for Ink Chamber

Referring again to FIG. 47, the ink refill opening to each chamber 38has a filter structure 40 to trap air bubbles or other contaminants. Airbubbles and solid contaminants in ink are detrimental to the MEMS nozzlestructures. The solid contaminants can obvious clog the nozzle openings,while air bubbles, being highly compressible, can absorb the pressurepulse from the actuator if they get trapped in the ink chamber. Thiseffectively disables the ejection of ink from the affected nozzle. Byproviding a filter structure 40 in the form of rows of obstructionsextending transverse to the flow direction through the opening, each rowbeing spaced such that they are out of registration with theobstructions in an adjacent row with respect to the flow direction, thecontaminants are not likely to enter the chamber 38 while the ink refillflow rate is not overly retarded. The rows are offset with respect toeach other and the induced turbulence has minimal effect on the nozzlerefill rate but the air bubbles or other contaminants follow arelatively tortuous flow path which increases the chance of them beingretained by the obstructions 40. The embodiment shown uses two rows ofobstructions 40 in the form of columns extending between the wafersubstrate and the nozzle plate.

Intercolour Surface Barriers in Multi Colour Inkjet Printhead

Turning now to FIG. 51, the exterior surface of the nozzle 56 is shownfor a unit cell such as that shown in FIG. 46 described above. Thenozzle apertures 26 are positioned directly above the heater elements(not shown) and a series of square-edged ink gutters 44 are formed inthe nozzle plate 56 above the ink conduit 23 (see FIG. 46).

Inkjet printers often have maintenance stations that cap the printheadwhen it's not in use. To remove excess ink from the nozzle plate, thecapper can be disengaged so that it peels off the exterior surface ofthe nozzle plate. This promotes the formation of a meniscus between thecapper surface and the exterior of the nozzle plate. Using contact anglehysteresis, which relates to the angle that the surface tension in themeniscus contacts the surface (for more detail, see the Applicant'sco-pending USSN (our docket FND007US) incorporated herein by reference),the majority of ink wetting the exterior of the nozzle plate can becollected and drawn along by the meniscus between the capper and nozzleplate. The ink is conveniently deposited as a large bead at the pointwhere the capper fully disengages from the nozzle plate. Unfortunately,some ink remains on the nozzle plate. If the printhead is a multi-colourprinthead, the residual ink left in or around a given nozzle aperture,may be a different colour than that ejected by the nozzle because themeniscus draws ink over the whole surface of the nozzle plate. Thecontamination of ink in one nozzle by ink from another nozzle can createvisible artefacts in the print.

Gutter formations 44 running transverse to the direction that the capperis peeled away from the nozzle plate will remove and retain some of theink in the meniscus. While the gutters do not collect all the ink in themeniscus, they do significantly reduce the level of nozzle contaminationof with different coloured ink.

Bubble Trap

Air bubbles entrained in the ink are very bad for printhead operation.Air, or rather gas in general, is highly compressible and can absorb thepressure pulse from the actuator. If a trapped bubble simply compressesin response to the actuator, ink will not eject from the nozzle. Trappedbubbles can be purged from the printhead with a forced flow of ink, butthe purged ink needs blotting and the forced flow could well introducefresh bubbles.

The embodiment shown in FIG. 46 has a bubble trap at the ink inlet 15.The trap is formed by a bubble retention structure 32 and a vent 36formed in the roof layer. The bubble retention structure is a series ofcolumns 32 spaced around the periphery of the inlet 15. As discussedabove, the ink priming features 18 have a dual purpose and convenientlyform part of the bubble retaining structure. In use, the ink permeabletrap directs gas bubbles to the vent where they vent to atmosphere. Bytrapping the bubbles at the ink inlets and directing them to a smallvent, they are effectively removed from the ink flow without any inkleakage.

Multiple Ink Inlet Flow Paths

Supplying ink to the nozzles via conduits extending from one side of thewafer to the other allows more of the wafer area (on the ink ejectionside) to have nozzles instead of complex ink distribution systems.However, deep etched, micron-scale holes through a wafer are prone toclogging from contaminants or air bubbles. This starves the nozzle(s)supplied by the affected inlet.

As best shown in FIG. 48, printheads according to the present inventionhave at least two ink inlets 15 supplying each chamber 38 via an inkconduit 23 between the nozzle plate and underlying wafer. Introducing anink conduit 23 that supplies several of the chambers 38, and is initself supplied by several ink inlets 15, reduces the chance thatnozzles will be starved of ink by inlet clogging. If one inlet 15 isclogged, the ink conduit will draw more ink from the other inlets in thewafer.

Droplet Stem Anchors

The droplet stem that attaches the ejected ink to the ink in the chamberimmediately prior to drop separation, can be a cause of dropmisdirection. FIGS. 55 to 59 show sequential stages of the drop ejectionprocess from a nozzle. In FIG. 55, the heater element 29 is rapidlyheated and vaporises the ink 64 in immediate contact with its surface tonucleate a bubble 66. This causes the ink meniscus 68 across the nozzleaperture 26 to start bulging outwardly.

In FIG. 56, the bubble 66 continues to grow as the heater element 29vaporises more of the ink 64 in the chamber 38. This pressure pulse fromthe growing bubble pushes the ink meniscus further out of the nozzleaperture 26. In FIG. 57, the bubble 66 continues to grow and the ejectedink starts to become a bulb 70 connected to the ink 64 in the chamber 38by a relatively thick droplet stem 72.

In FIG. 58, the bubble has grown to the point where it vents toatmosphere through the nozzle aperture 26. This is an importantmechanism for avoiding cavitation corrosion of the heater element 29.Cavitation corrosion occurs when a bubble collapses back to a singlepoint on the heater element surface. As the bubble reaches thesingularity of a collapse point, the surface tension creates severehydraulic forces that can abrade the heater material. By venting thebubble, there is no collapse point on the heater element.

As shown in FIG. 58, when the bubble vents, the droplet stem 72 canattach itself to a point 74 on the nozzle rim. As the attachment point74 is not on the centre-line 76 of the nozzle aperture 26, the ink bulb70 is deflected 78 away from the centre-line because of the surfacetension's tendency to reduce surface area.

Referring to FIG. 59, the stem 52 eventually breaks and the ink drop 80forms and continues on its trajectory to the print media. However, themisdirection 78 remains for the ink drop 80 as well as any satellitedrops 82. The vented bubble has become an extended ink meniscus thathelps to draw ink back into the chamber as it contracts to the nozzleaperture 26.

FIGS. 60-67 show nozzle designs with droplet stem anchors thatpositively locate where the droplet stem attaches. Knowing where thestem will attach reduces the misdirection, or in some cases, controlsthe misdirection so that all nozzles are misdirected in the samedirection by roughly the same amount. However, the droplet stem anchorscan also perform secondary functions and these will now be discussedbelow.

Combining Ink Ejected from Adjacent Actuators

Referring to FIGS. 60 and 61, the nozzle design shown has two actuators29 ejecting ink through a single oval shaped nozzle 25. The actuatorsare both heater elements connected in series for simultaneous actuationand ejection. Both actuators 29 are part of a single beam of heatermaterial such as TiAlN which is suspended at its ends and at it midpoint. Both heater elements 29 have a tapered section 86 whereelectrical resistance is at a maximum. During actuation, the vapourbubbles initiate at these maximum resistance sections or ‘hotspots’ 86.

The ink covering both heater elements 29 is connected by the slots 88.The slots can be dimensioned so that they damp fluidic cross talk to theextent that the heater elements are in two separate ink chambers, orthey can be large enough to that both elements 29 are considered to bein the same chamber 38. The heater elements 29 are positioned relativeto the droplet stem anchor 84 so that as the ink ejected by eachactuator forms a bulb attached by a stem, the ink surface tension,seeking to occupy the least surface area, will attach the stem to theanchor in preference to any other point on the nozzle rim 25. As thehotspots 86 are on diametrically opposed sides of the anchor 84, thebulbs of ink attached to respective droplet stems will be misdirectedtoward each other. Eventually they meet directly above the anchor andthe opposing misdirections cancel each other out, or at least, theresultant misdirection is very small.

Quadrupolar Actuation

FIGS. 62-65 show several embodiments of nozzles with quadrupolaractuation. Quadrupolar actuation initiates the pressure pulse atpositions in the ink chamber that are symmetrical about two orthogonalaxes. As the pulses converge within the chamber, the symmetry about twoaxes pushes the ink in a direction that is normal to both axes, at leastin the ideal case. In reality, slight asymmetries mean the dropdirection may be not be exactly normal, but it will typically be muchcloser than if the pressure pulse initiated from a single point in thechamber.

Referring to FIG. 62, the unit cell shows two nozzles 25 in respectivechambers 38, each having a quadrupole thermal actuator 12. The heaterelement portion 29 of each actuator 12 is shaped similar to the Greekletter ‘theta’. Each actuator has two semi-circular current paths 90between the contacts 28. A central bar 94 extends between the mid pointsof each current path. The entire theta-shaped structure is suspended inthe chamber 38 to minimise heat dissipation into the wafer substrate andmaximise heater transfer to the ink.

The central bar 94 serves multiple purposes. Firstly, it provides theheater element with structural rigidity and bracing. Without it, thecyclical heating and cooling of the semi-circular current paths wouldcause some buckling into or out of the page of FIG. 62. This could beaddressed by supporting the semi-circles on the chamber floor, or evenby a single support at each mid-point. However, this increases contactwith the underlying wafer substrate and therefore increases heatdissipation. The central bar 94 provides resistance to buckling whilekeeping the heater element suspended within the chamber.

The central bar 94 also provides a ‘cold spot’ 92 at the mid-point ofeach semi-circle. The thermal mass of the bar provides a small heat sinkso the junction between the bar and the semi-circular current path heatsto bubble nucleation temperature more slowly than the sections eitherside of the junction. Likewise, the contacts 28 act as heat sinks sobubble nucleation is directed to the middle of the arc between thecontact and the junction with the central car 94. This ensures that thevapour bubbles nucleate at four positions on the theta shape and thatthese positions have quadrupole symmetry about two orthogonal axes.

Finally, the central bar also provides a droplet stem anchor foradditional control of misdirection. If the position of the central bar94 below the nozzle 25 is such that the area of the surface tension isminimised if the droplet stem attaches to the bar instead of a point onthe nozzle 25, then the drop trajectory will be more closely alignedwith the central axis extending normal to the nozzle aperture 26.

In FIGS. 63 and 64, the central bar 94 has a latch point 96 for locatingthe base of the droplet stem. The latch point is simply a surfaceirregularity that the surface tension of the ink can ‘pin’ itself to. Ifthe central bar 94 is not parallel to the plane of the nozzle aperture26, or there is some asymmetry in the position of the bubble nucleationsites, the droplet stem may latch to an off centre part of the centrebar 94. A surface irregularity 96 on the central bar 94 tends to snag onthe surface tension of the droplet stem and anchor it to the middle ofthe bar. The surface irregularity 96 can be a sudden reduction in crosssection as shown in FIG. 63, or a boss such as that shown in FIG. 64. Ineither case, the droplet stem originates from the middle of the centralbar 94 and so any misdirection in the drop trajectory is minimised.

Dual Bar, Four Kink, Heater Element

FIG. 65 shows another quadrupole thermal actuator 12. Again it has twocurrent paths 90 provided by separate beams extending between thecontacts 28. For clarity, the other features of the unit cell have beenomitted.

The beams 90 are suspended in the chamber 38 to minimise heatdissipation into the wafer substrate and each beam has two tight radiuscurves or kinks 98, between curves of larger radius 96. In thisembodiment, the tight radius kinks 98 act as hotspots where the vapourbubbles nucleate. This is because the current flow around the kinks 98will concentrate towards the radially inner side of the element 102 andaway from the outside radius 100. This acts like a localised reductionin cross section which increases the resistance at these points. In thelarge radius curves 96, the difference in current density between theinside edge and the outside edge is much less so the increase inresistance is small compared to that in the tight kinks 98.

The tight kinks 98 have a relatively low bending resistance so thelongitudinal expansion of the beam 90 during actuation is accommodatedwithout buckling inot or out of the plane of the page. This makes theposition of the hotspots in the chamber 38 relatively stable therebymaintaining the quadrupole symmetry and minimising drop misdirection.

Rectifying Valve at Ink Chamber Inlet

The unit cell shown in FIG. 66 has a rectifying valve 106 at the inkrefill aperture 104 to each chamber 38. The particular rectifying valveshown is known as a Tesla valve. A rectifying valve provides lesshydraulic resistance to ink flowing into the chamber 38 than ink flowingout of the chamber. This can be used to reduce fluidic cross talkbetween chambers 38, while not retarding ink refill times (and thereforeprint speeds).

For the purposes of this example, the heater element 29 is a simple beamsuspended in the chamber 38 between the contacts 28. Also for clarity,the nozzle rim has been omitted, however the skilled worker willappreciate that it is centrally disposed over the heater element 29.Alternatively, the chambers 38 could have several nozzles each, asdiscussed above.

The chambers 38 are supplied with ink from the ink inlet 15 via thelateral ink conduit 23. The Tesla valve 106 at each refill aperture 104has a main conduit 108 between a pair of smaller secondary conduits 110.As ink flows into the chamber 38, there is little resistance to the flowthrough the main conduit 108 other than fluidic drag against the wallsof the conduit itself. The upstream openings of the secondary conduits110 do not face into the flow so little of the main flow is divertedinto them. The downstream openings direct any flow parallel and adjacentto the flow from the main conduit 108 downstream opening. Therefore, thesecondary conduits 110 have negligible impact on ink flow into thechamber 38.

Upon actuation, the pressure pulse can create a back flow of ink out ofthe chamber 38 and back into the lateral ink conduit 23. Back flow isdetrimental to drop ejection as it uses some of the energy from thepressure pulse. The back flow can also create fluidic cross talk thataffects the ejection characteristics of adjacent chambers.

The Tesla valve 106 resists any back flow by using flow from thesecondary conduits 110 to constrict flow through the main conduit 108.During back flow, the upstream openings of the secondary conduits 110are facing the flow direction. So to is the upstream opening to the mainconduit 108. The pressure pulse forces ink along the main and secondaryconduits however, the downstream openings of the secondary conduits 110direct their ink flow across and counter to the main flow direction.These conflicting flows create turbulence and a hydraulic constrictionin the main conduit 108. Hence back flow through the main conduit 108and the secondary conduits 110 is stifled. With a high resistance toback flow, a greater portion of the pressure pulse is used to eject theink drop through the nozzle and fluidic cross talk is reduced.

Controlled Drop Misdirection

FIG. 67 is a schematic perspective of a nozzle with controlled dropmisdirection. This is a different approach to minimising the dropmisdirection as discussed above. By intentionally misdirecting the dropsejected by every nozzle in the array by a controlled amount, the printedimage is equivalent to one from a minimised drop misdirection printhead(albeit slightly offset from the nozzle array).

As with minimising drop misdirection, this approach uses a droplet stemanchor 74 is positioned so that the droplet stem will attach to it inpreference to any other point on the nozzle rim 25 or heater element 29.However, in nozzle designs that do not allow the drop to formsymmetrically around the droplet stem anchor, so the drop trajectory isnot normal to the plane of the nozzle aperture, the anchor can bepositioned at a point that will cause a known misdirection that is thesame magnitude and direction as every other nozzle in the array.

The embodiment shown in FIG. 67 provides a droplet stem anchor at theend of a lateral spur 112 extending into the nozzle aperture 26 from theside of the nozzle rim 25. This nozzles uses a simple suspended beamheater element 29 which is easier to deposit and etch than a thetaheater (described above), but still controls drop misdirection with adroplet stem anchor. It will be appreciated that the spur 112 is anobstruction that deflects the drop from the normal trajectory. However,if all the spurs in the nozzle array are parallel and have the sameposition relative to the heater element, the misdirection across thewhole array will be uniform.

Although the invention is described above with reference to specificembodiments, it will be understood by those skilled in the art that theinvention may be embodied in many other forms.

1. An inkjet printhead comprising an array of ink chambers, each inkchamber comprising: a plurality of nozzles; and a thermal actuator forgenerating vapour bubbles to eject ink through the nozzles, the thermalactuator comprising a pair of contacts and at least two parallel currentpaths between the contacts, each of the current paths having a pluralityof heater elements connected in series.
 2. An inkjet printhead accordingto claim 1, wherein the heater elements are suspended within thechamber.
 3. An inkjet printhead according to claim 1, wherein respectiveheater elements are associated with respective nozzles.
 4. An inkjetprinthead according to claim 1 wherein the heater elements nucleatebubbles simultaneously.
 5. An inkjet printhead according to claim 1wherein the thermal actuator has a cross bracing structure extendingbetween intermediate points on the parallel current paths.
 6. An inkjetprinthead according to claim 5 wherein the cross bracing structureprovides increased thermal inertia to the current paths where the crossbracing structure connects to the current paths.
 7. An inkjet printheadaccording to claim 5 wherein the cross bracing structure provides adroplet stem anchor.
 8. An inkjet printhead according to claim 1 whereinthe thermal actuator is formed from TiAlN.
 9. An inkjet printheadaccording to claim 1 wherein each of the ink chambers has two nozzles.10. An inkjet printhead according to claim 1 wherein the nozzles in eachchamber are arranged in a line parallel to the length of the heaterelement, with the central axes of the nozzles being regularly spacedalong the heater element.
 11. An inkjet printhead according to claim 1wherein the nozzles are elliptical.
 12. An inkjet printhead according toclaim 11 wherein the major axes of the elliptical nozzles are aligned.